The present invention relates to magnetoresistive devices for reading out data recorded on magnetic memory media, and also to magnetoresistive sensors, magnetoresistive sensor systems and magnetic memory systems using the same.
Magnetic reading transducers which are called magnetoresistive (MR) sensors or MR heads are well known in the art. These magnetic reading transducers feature that data can be read out from a magnetic memory medium surface at high linear density. The MR sensors detect magnetic field signal via a resistance change, which is a function of the intensity and direction of a magnetic flux sensed by a reading device. Such prior art MR sensors are operable on the basis of anisotropic magnetoresistive (AMR) effect, i.e., changes in a component of the resistance of the reading device in proportion to the cosine of the angle between the magnetizing direction and the direction of a sense current through the device. The AMR effect is described in greater details in D. A. Thompson, "Memory, Storage and Related Applications", IEEE Trans. on Mag. MAG-11, p. 1039, 1975. In many magnetic head utilizing the AMR effect, a vertical bias is applied in order to suppress the Barkhousen noise. In some applications, anti-ferromagnetic materials such as FeMn, NiMn and nickel oxide are used as vertical bias application material.
Up to date, a more pronounced magnetoresistive effect has been reported, in which changes in the resistance of a laminate magnetic sensor are attributable to the spin-dependent transfer of conductive electrons between two magnetic layers via an intervening non-magnetic layer and also to accompanying spin-dependent dispersions of conductive electrons in the interfaces between the adjacent layers. This magnetoresistive effect is also called "giant magnetoresistive effect", "spin valve effect" and so forth. Such MR sensors are made of adequate materials, and they have improved sensitivity and provide great resistance changes compared to those observed with sensors utilizing the AMR effect. In this type of MR sensor, the plane resistance between a pair of ferromagnetic layers spaced apart by a non-magnetic layer is changed in proportion to the cosine of the angle between the magnetizing directions of the two layers.
Japanese Laid-Open Patent Publication No. 2-61572 discloses a magnetic laminate structure providing great MR changes which are attributable to anti-parallel array of magnetization in a magnetic layer. The publication mentions ferromagnetic transition metals and alloys thereof as materials capable of being used for the laminate structure. The publication further discloses a structure, which has an anti-ferromagnetic layer provided for at least one of two ferromagnetic layers spaced apart by an intervening layer, and also teaches that FeMn is adequately used for the anti-ferromagnetic layer.
Japanese Laid-Open Patent Publication No. 4-358310 discloses an MR sensor, which has two ferromagnetic thin-film layers partitioned by a non-magnetic metal thin-film layer. In this MR sensor, when the applied magnetic field is zero, the magnetizing directions of the two ferromagnetic thin-film layers are perpendicular to each other, and the resistance between the two non-coupled ferromagnetic layers is changed in proportion to the cosine of the angle between the magnetizing directions of the two layers and independent of the direction of current passing through the sensor.
Japanese Laid-Open Patent Publication No. 6-203340 discloses an MR sensor, which again has two ferromagnetic thin-film layers spaced apart by a non-magnetic metal thin film. This MR sensor is based on the above effect that when the external applied magnetic field is zero, the magnetizing direction of an adjacent anti-ferromagnetic layer is perpendicular to the other ferromagnetic layer.
Japanese Laid-Open Patent Publication No. 7-262529 discloses a magnetoresistive device, which is a spin valve having a structure including a first magnetic layer, a non-magnetic layer, a second magnetic layer and an anti-ferromagnetic layer, these layers being formed in the mentioned order. Particularly, the first and second magnetic layers are formed by using CoZrNb, CoZrMo, FeSiAl, FeSi or NiFe with or without addition of Cr, Mn, Pt, Ni, Cu, Ag, Al, Ti, Fe, Co or Zn.
Japanese Laid-Open Patent Publication No. 7-202292 discloses a magnetoresistive film, which includes a plurality of magnetic thin films laminated on a substrate via a nonmagnetic film. An anti-ferromagnetic thin film is provided adjacent one of two soft magnetic thin films adjacent to each other via a non-magnetic thin film. In this magnetoresistive film, the bias magnetic field Hr applied to the anti-ferromagnetic film and the coercive force Hc2 of the other soft magnetic thin film are related to be Hc2&lt;Hr. The anti-ferromagnetic material is a member of an alloy of two or more members of the group consisting of NiO, CoO, FeO, Fe.sub.2 O.sub.3, MnO and Cr.
Japanese Patent Applications No. 6-214837 and 6-269524 disclose the above magnetoresistive film, wherein the anti-ferromagnetic material comprises at least two superlattice members selected from the group consisting of NiO, Ni.sub.x Co.sub.1-x O and CoO.
Japanese Patent Application No. 7-11354 discloses the above magnetoresistive film, wherein the anti-ferromagnetic material comprises at least two super-lattice members selected from the group consisting of NiO, Ni.sub.x Co.sub.1-x O (X=0.1 to 0.9) and CoO, the atomic number ratio of Ni to Co in the super-lattices being 1.0 or above.
Japanese Patent Application No. 7-136670 discloses the above magnetroresistance effect film, wherein the anti-ferromagnetic material is provided as a two-layer film obtained by laminating CoO of 10 to 40 angstroms on NiO.
The Japanese Laid-Open Patent Publication No. 7-262529 discloses examples of the magnetoresistive device, in which the under layer is formed as a Si.sub.3 N.sub.4 (50 nm thick)/Hf (5 nm thick)/Ta (5 nm thick).
In the prior art magnetoresistive device having the basic structure by laminating an under layer, a free magnetic layer, a non-magnetic layer, a fixed magnetic layer and an anti-ferromagnetic layer in the mentioned order, the resistance change rate is greatly reduced by annealing at 200.degree. C. or above. Many magnetoresistive devices of this type require a heat treatment at 200.degree. C. or above in order to obtain an exchange coupled field provided form the anti-ferromagnetic layer to the fixed magnetic layer. Such a heat treatment, however, results in deterioration of the interfaces between the free magnetic and non-magnetic layers and between the non-magnetic and fixed magnetic layers, thus reducing the magnetoresistive change rate.
Even where an anti-ferromagnetic magnetic layer of the type not requiring any heat treatment is used, a process of hardening the resist of a write head section is indispensable when actually manufacturing a recording/reproducing head. This process requires a heat treatment at a temperature of 200.degree. C. or above. In the stage when the actual head has been obtained, therefore, the resistance change rate of the magnetoresistive film has been greatly reduced, thus posing a problem of failure of obtaining a design output.